The principal objectives in petroleum refining are to separate the components of crude oils of varying compositions and qualities into components with specific utility and, in addition, to increase the yield of relatively high value components from the relatively larger proportion of lower value components in the crude itself. The quality of crude oils which will be processed by refineries is expected to deteriorate progressively in the future while the demand for high boiling and residual products such as heavy fuel oil is expected to decrease. In addition, the effect of laws regulating the quality of petroleum fuels will necessitate the production of higher quality napthas and middle distillates. In particular, regulations controlling the permissible lead content in motor gasoline will necessitate the production of high octane naphtha and requirements for low sulfur middle distillates, especially diesel fuel and home heating oil is expected to lead to more stringent specifications for those products also. At the same time, the decreased demand for heavy fuel oil previously mentioned will make it difficult to supply the improved lower boiling products out of the lower boiling fractions of the crude, commonly referred to as the "top of the barrel". This means that the relatively higher boiling fractions must be processed to satisfy the expected demands but at the same time, the cost of processing must be contained within reasonable bounds. For this reason, although hydroprocessing is expected to play a major role in order to convert the relatively higher boiling fractions in the crude to the more valuable products, the processes should be made as efficient as possible in order to minimize hydrogen consumption. As is well known, hydrogen plants are expensive to construct and also to operate. Thus, there is a continuing need to develop improved processing techniques for converting residua, gas oils and other high boiling materials to naphtha, middle distillates such as diesel fuel, jet fuel, home heating oil, light fuel oil and the like.
Hydrocracking is an established refining process in which a great deal of interest has developed in recent years. This interest has been caused by several factors including the demand for gasoline compared to middle distillates and the necessary for dealing with large volumes of dealkylated refractory effluents from catalytic cracking plants at a time when there has been a decrease in the demand for the fuel oil products into which these refractory materials were previously incorporated. The hydrocracking process is, unlike catalytic cracking, able to deal effectively with these otherwise refractory materials. In addition, by-product hydrogen has become available in large amounts from catalytic reforming operations so that a large proportion of the hydrogen required for hydrocracking can be readily supplied. A further factor which has favored the development of hydrocracking is that over the last 20 years, the use of zeolite cracking catalysts has become predominant and the cycle oils which result from cracking operation with zeolite catalysts tend to be highly aromatic and to be excellent feedstocks for hydrocracking processes.
The hydrocracking feedstock is invariably hydrotreated before being passed to the hydrocracker in order to remove sulfur and nitrogen compounds as well as metals and, in addition, to saturate olefins and to effect a partial saturation of aromatics. The sulfur, nitrogen and oxygen compounds may be removed as inorganic sulfur, nitrogen and water prior to hydrocracking although interstage separation may be omitted, as in the Unicracking-JHC process. Although the presence of large quantities of ammonia may result in a suppression of cracking activity in the subsequent hydrocracking step this may be offset by an increase in the severity of the hydrocracking operation.
In the hydrotreater, a number of different hydrogenation reactions take place including olefin and aromatic ring saturation but the severity of the operation is limited so as to minimize cracking. The hydrotreated feed is then passed to the hydrocracker in which various cracking and hydrogenation reactions occur. The cracking reactions provide olefins for hydrogenation while hydrogenation in turn provides heat for cracking since the hydrogenation reactions are exothermic while the cracking reactions are endothermic; the reaction generally proceeds with an excessive heat generated because the amount of heat released by the exothermic hydrogenation reactions is much greater than the amount of heat consumed by the endothermic cracking reactions. This surplus of heat causes the reactor temperature to increase and accelerate the reaction rate but control is provided by the use of hydrogen quench.
Conventional hydrocracking catalysts combine an acidic function and a hydrogenation function. The acidic function in the catalyst is provided by a porous solid carrier such as alumina, silica-alumina or by a composite of a crystalline zeolite such as faujasite, zeolite X, zeolite Y or mordenite with an amorphous carrier such as silica-alumina. The use of a porous solid with a relatively large pore size in excess of 7A is generally required because the bulky, polycyclic aromatic compounds which constitute a major portion of the typical feedstock require pore sizes of this magnitude in order to gain access to the internal pore structure of the catalyst where the bulk of the cracking reactions takes place.
The hydrogenation function in the hydrocracking catalyst is provided by a transition metal or combination of metals. Noble metals of Group VIIIA of the Periodic Table, especially platinum or paladium may be used but generally, base metals of Groups IVA, VIA and VIIIA are preferred because of their lower cost and relatively greater resistance to the effects of poisoning by contaminants (the Periodic Table used in this specification is the table approved by IUPAC as shown, for example, in the chart of the Fisher Scientific Company, catalog No. 5-702-10). The preferred base metals for use as hydrogenation components are chromium, molybdenum, tungsten, cobalt and nickel and combinations of metals such as nickel-molybdenum, cobalt-molybdenum, cobalt-nickel, nickel-tungsten, cobalt-nickel-molybdenum and nickel-tungsten-titanium have been shown to be very effective and useful.
One characteristic of the conventional hydrocracking catalysts is that they tend to be naphtha directing, that is, they tend to favor the production of naphthas, typically boiling below about 165.degree. C. (about 330.degree. F.) rather than middle distillates such as jet fuel and diesel fuel, typically boiling above about 165.degree. C. (about 330.degree. F.), usually in the range 165.degree. to 345.degree. C. (about 330.degree. to 650.degree. F.). However, the yield of middle distillates may be relatively increased by operating under appropriate conditions. For example, U.S. Pat. No. 4,435,275 describes a process for producing low sulfur distillates by operating the hydrotreating-hydrocracking process without interstage separation and at relatively low pressures, typically below about 7,000 kPa (about 1,000 psig). The middle distillate product from this process is an excellent low sulfur fuel oil but it is generally unsatisfactory for use as a jet fuel because of its high aromatic content; this high aromatic content also makes it unsuitable for use as a diesel fuel on its own but it may be used as a blending component for diesel fuels if other base stocks of higher certane number are available. Conversion is maintained at a relatively low level in order to obtain extended catalyst life between successive regenerations under the low hydrogen pressures used. Relatively small quantities of naphtha are produced but the naphtha which is obtained is an excellent reformer feed because of its high cycloparaffin content, itself a consequence of operating under relatively low hydrogen pressure so that complete saturation of aromatics is avoided.
The use of highly siliceous zeolites as the acidic component of the hydrocracking catalyst will also favor the production of distillates at the expense of naptha, as described in U.S. patent applications Ser. No. 744,897, filed June 17, 1985 and its counterpart EU No. 98,040.
In conventional hydrocracking processes for producing middle distillates, especially jet fuels, from aromatic refinery streams such as catalytic cracking cycle oils it has generally been necessary to employ high pressure hydrotreating typically about 2000 psig to saturate the aromatics present in the fed so as to promote cracking and to ensure that a predominantly paraffinic/naphthenic product is obtained. The hydrocracked bottoms fraction is usually recycled to extinction even though it is highly paraffinic (because of the aromatic-selective character of the catalyst) and could form the basis for a paraffinic lube stock of higher value than the distillate produced by cracking it. Thus, the conventional fuels hydrocracker operating with a cycle oil feed not only is demanding in terms of operating requirements (high hydrogen pressure) but also degrades a potentially useful and valuable product.
A significant departure in hydrocracking is described in U.S. patent application Ser. No. 379,421 and its counterpart EU No. 94,827. The catalyst used in the process is zeolite beta, a zeolite found to have a combination of unique and highly useful properties. Zeolite beta, in contrast to conventional hydrocracking catalysts, has the ability to attack paraffins in the feed in preference to the aromatics. The effect of this is to reduce the paraffin content of the unconverted fraction in the effluent from the hydrocracker so that it has a relatively low pour point. By contrast, conventional hydrocracking catalysts such as the large pore size amorphous materials and crystalline aluminosilicates previously mentioned, are aromatic selective and tend to remove the aromatics from the hydrocracking feed in preference to the paraffins. This results in a net concentration of high molecular weight, waxy paraffins in the unconverted fraction so that the higher boiling fractions from the hydrocracker retain a relatively high pour point (because of the high concentration of waxy paraffins) although the viscosity may be reduced (because of the hydrocracking of the aromatics present in the feed). The high pour point in the unconverted fraction has generally meant that the middle distillates from conventional hydrocracking processes are pour point limited rather than end point limited. The specifications for products such as light fuel oil (LFO), jet fuel and diesel fuel generally specify a minimum initial boiling point (IBP) for safety reasons but end point limitations usually arise from the necessity of ensuring adequate product fluidity rather than from any actual need for an end point limitation in itself. In addition, the pour point requirements which are imposed effectively impose an end point limitation of about 345.degree. C. (about 650.degree. F.) with conventional processing techniques because inclusion of higher boiling fractions including significant quantities of paraffins will raise the pour point above the limit set by the specification. When zeolite beta is used as the hydrocracking catalyst, however, the lower pour point of the unconverted fraction enables the end point for the middle distillates to be extended so that the volume of the distillate pool can be increased. Thus, the use of zeolite beta as the acidic component of the hydrocracking catalyst effectively increases the yield of the more valuable components by reason of its paraffin selective catalytic properties.
Another characteristic of zeolite beta is that it affects removal of waxy paraffinic components from the feed by isomerization as well as by conventional cracking reactions. The waxy paraffinic components, comprising straight chain end paraffins and slightly branched chain paraffins, especially the monomethyl paraffins, are isomerized by zeolite beta to form iso-paraffins which form excellent lubricant bases because the iso-paraffins possess the high viscosity index characteristic of paraffins without the high pour point values which are characteristic of the more waxy paraffins. A process employing this property of zeolite beta for dewaxing feeds to produce a low pour point distillates and gas oil is described in U.S. Pat. No. 4,419,220.
In spite of the potential of improvement offered by the use of zeolite beta as a hydroprocessing catalyst, both in dewaxing processes under conditions of limited severity and conversion and in hydrocracking processes where a significant bulk conversion occurs, the need for improving the yield and quality of the gasoline and middle distillate pool perists. The unique characteristics of zeolite beta and the desirable results demonstrated so far indicate that further improvements may be achieved with it, particularly by integrating its use into other refining schemes which enable its unique properties to be best exploited.
In considering the problems encountered with jet fuel production, the limitations imposed by the moderate pressure hydrocracking process become apparent: the middle distillate product which is obtained is relatively aromatic in character because it is not possible to carry out extensive aromatics saturation at the relatively how hydrogen pressures used. Conversely, the bottoms fraction is relatively paraffinic because of the aromatic-selective character of the catalysts used. Because it is relatively paraffinic, as well as being of high boiling point this bottoms fraction commends itself for consideration as a lubestock or, at least, as the starting point in lubricant stock production. However, it is generally of high pour point and freeze point because of the presence of waxy paraffinic components (n-paraffins and slightly branched chain pariffins, especially mono-methyl paraffins) so that dewaxing is necessary. The present processing scheme effectively integrates jet fuel and middle distillate production by moderate pressure hydrocracking with a catalytic lube production process so as to maximize the production of both types of product and to exploit most effectively the characteristics of the moderate pressure hydrocracking process and the catalystic isomerization dewaxing process using dewaxing catalysts based on zeolite beta.